1. Field of the Invention
This invention relates to a moving turbine blade, and more particularly, one useful when applied to an axial flow impulse turbine.
2. Description of the Related Art
FIG. 5 is a schematic view showing moving turbine blades, along with stationary turbine blades, of an axial flow impulse turbine according to an earlier technology. As shown in the drawing, a multiplicity of moving turbine blades 1 are disposed in a circumferential direction of an impeller (not shown). A multiplicity of stationary blades 2 are fixed to a casing (not shown) of the axial flow impulse turbine. The stationary blades 2 function as nozzles for supplying a high velocity, high pressure fluid (e.g. steam) to the moving turbine blades 1.
An analysis of flow velocity in this type of axial flow impulse turbine has now shown the occurrence of an important phenomenon. With this type of turbine, conventional knowledge has been that a slow flow velocity region extending in a band form, called a stationary blade wake 3 (a dotted portion in the drawing), is formed behind a rear edge 2a of the stationary blade 2. A recent finding is that each time the moving turbine blade 1 cuts across the stationary blade wake 3 upon rotation of the turbine, a high velocity region (a cross-hatching in the drawing) 4 of a sharply quickening fluid occurs on a dorsal surface portion 1a of the moving turbine blade 1. The mechanism for formation of this region may be as follows: The stationary blade wake 3 functions as an effective wall against a main stream with a high flow velocity. Consequently, as the moving turbine blade 1 approaches the stationary blade wake 3 in accordance with rotational movement of the moving turbine blade 1 (the direction of the rotational movement is indicated by an arrow A in the drawing), a throat of a passageway-effectively forms between the stationary blade wake 3 and the moving turbine blade 1. As a result, the high velocity region 4 of a sharply quickening fluid occurs on the dorsal surface portion 1aof the moving turbine blade 1 with the passage of time. Such a stationary blade wake 3 is formed behind each of the stationary blades 2, and the high velocity region 4 is also formed in correspondence with each stationary blade wake 3. However, only one stationary blade wake 3 and only one high velocity region 4 are shown in the drawing as representatives.
When the above-described unsteady high velocity region 4, where the flow velocity sharply increases at the instant of approach to the stationary blade wake 3, is formed on the dorsal surface portion 1a of the moving turbine blade 1, a turbine loss at this site is great. This is because if the wall stands in the passageway of the fluid, friction corresponding to the difference in flow velocity between the high velocity region and the low velocity region appears, and the kinetic energy of the fluid changes into heat owing to this friction. That is, a total pressure loss occurs. Thus, the efficiency of the turbine declines.
The present invention has been accomplished in light of the foregoing problems with the earlier technology. The object of the invention is to provide a moving turbine blade which can contribute to increasing the efficiency of a turbine while suppressing an unsteady, sharp increase in flow velocity.
To attain the above object, the inventors investigated conditions for formation of a marked high velocity region 41 and obtained the following findings: The shape of the stationary blade wake 3 is determined simply by the shape of the stationary blade 2. The moving turbine blade 1, on the other hand, is configured from the aspect that a smooth flow velocity distribution in a range from a front edge 1b to a rear edge 1d of the moving turbine blade 1 is ensured based on the outlet angle of the fluid flowing out of the stationary blade 2. From this aspect, the approximate inlet angle and the approximate shapes of the dorsal surface portion 1a and a ventral surface portion 1c are determined. As a result, with the moving turbine blade 1 according to the earlier technology, the dorsal surface portion 1a at the site of the front edge 1b of the moving turbine blade 1 is shaped to be parallel to the stationary blade wake 3. This shaping of the dorsal surface portion 1a of the moving turbine blade 1 to be parallel to the stationary blade wake 3 may be the major cause of the unsteady, sharp increase in the flow velocity. When the dorsal surface portion 1a is shaped to be parallel to the stationary blade wake 3, a throat of the passageway is formed, most prominently, between the stationary blade wake 3 and the dorsal surface portion 1a of the moving turbine blade 1.
The features of the present invention based on the foregoing findings are characterized by the following aspects 1) to 5):
1) A moving turbine blade in a turbine having a multiplicity of moving turbine blades disposed in a circumferential direction of an impeller, the moving turbine blades being acted on by a fluid, which has left stationary blades as fixed blades, to transmit a rotating force to the impeller, wherein:
a shape of a dorsal surface portion, at a front edge and in a portion adjacent thereto, of the moving turbine blade is a chamfered shape so as not to be parallel to a stationary blade wake.
According to the above aspect of the invention, the shape of the dorsal surface portion at the front edge of the moving turbine blade can be displaced from the stationary blade wake. Thus, it becomes possible to widen a passageway formed between the dorsal surface portion at the front edge of the moving turbine blade and the stationary blade wake when the moving turbine blade cuts across the stationary blade wake upon its rotational movement. Hence, an unsteady increase of the flow velocity on the dorsal surface portion can be suppressed. Consequently, even when the moving turbine blade periodically cuts across the stationary blade wake in accordance with movement of the moving turbine blade, it becomes possible to remove a partial high velocity region of the flow velocity, eliminate a total pressure loss at this site, and contribute to increasing the efficiency of the turbine.
2) In the moving turbine blade described in the aspect 1) above, when an angle, which a tangent to the dorsal surface portion at the front edge of the moving turbine blade makes with a straight line perpendicular to a rotating shaft of the turbine, is designated as xcex8, and a geometrical outlet angle of the stationary blade is designated as xcex1N, xcex8 is in the following relationship:
xcex1N+2xc2x0 less than xcex8 less than xcex1N+12xc2x0
According to this aspect, not only the actions described in the aspect 1) have been obtained, but also the upper limit value of xcex8 has been restricted. Thus, a geometrical relationship, such as the inlet angle of the moving turbine blade relative to the outlet angle of the stationary blade, can be ensured optimally. Consequently, the moving turbine blades can contribute to increasing the efficiency of the turbine, without sacrificing other characteristics.
3) In the moving turbine blade described in the aspect 1) or 2) above, when a maximum blade thickness of the moving turbine blade is designated as Tmax, and a blade width, which is a distance between the front edge and a rear edge of the moving turbine blade, is designated as W, Tmax/W is in the following relationship:
0.33 less than Tmax/W less than 0.42
According to this aspect, not only the actions described in the aspects 1) and 2) have been obtained, but also the blade shape of the moving turbine blade is thin-walled. Thus, the passageway between the adjacent moving turbine blades is widened. The average flow velocity at this site can be decreased. Consequently, a high velocity region of the flow velocity between the stationary blade wake and the dorsal surface portion of the moving turbine blade can be removed further satisfactorily, and a further increase in the turbine efficiency can be facilitated.
4) In the moving turbine blade described in the aspect 1) or 2) above, when an angle, which a tangent to a ventral surface portion at the front edge of the moving turbine blade makes with a tangent to the dorsal surface portion, is designated as xcex2inc, xcex2inc is in the following relationship:
13xc2x0 less than xcex2inc less than 27xc2x0
According to this aspect, not only the actions described in the aspects 1) and 2) have been obtained, but also the moving turbine blade has a small blade thickness in the vicinity of the front edge where the flow velocity is particularly increased because of the stationary blade wake. Thus, the passageway between the adjacent moving turbine blades is widened. The average flow velocity at this site can be decreased. Consequently, a high velocity region of the flow velocity between the stationary blade wake and the dorsal surface portion of the moving turbine blade can be removed further satisfactorily, and a further increase in the turbine efficiency can be facilitated.
5) In the moving turbine blade described in the aspect 1) or 2) above, when a maximum blade thickness of the moving turbine blade is designated as Tmax, and a blade width, which is a distance between the front edge and a rear edge of the moving turbine blade, is designated as W, Tmax/W is in the following relationship:
0.33 less than Tmax/W less than 0.42,
and when an angle, which a tangent to a ventral surface portion at the front edge of the moving turbine blade makes with a tangent to the dorsal surface portion, is designated as xcex2inc, xcex2inc is in the following relationship:
13xc2x0 less than xcex2inc less than 27xc2x0
According to this aspect, the superimposition of the actions described in the aspect 1) or 2) and the aspects 3) and 4) is obtained. Consequently, the turbine efficiency can be increased most remarkably.